# Lec3 - Physics 101 Today: Chap 4 - Newtons Second Law Will...

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Today: Chap 4 - Newton’s Second Law Will establish a relationship between force (chap 2) and acceleration (chap. 3). Physics 101

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Mass and Weight Mass = measure of inertia of object. Quantity of matter in the object. Denote m . Weight = force upon an object due to gravity Recall: inertia measures resistance to any effort made to change its motion In fact, weight = mg Often weight and mass are used interchangeably in every- day life, but in physics, there is a fundamental difference. Eg. In outer space, there is no gravity so everything has zero weight. But, things still have mass. Shaking an object back and forth gives sense of how massive it is because you sense the inertia of it.
Mass and Weight continued Units: Standard unit for mass is kilogram, kg. Standard unit for weight is Newton (since it’s a force) (commonly, pound) Note mass is an intrinsic property of an object - eg. it doesn’t depend on where it is, whereas weight does depend on location (eg less on moon than on earth…)

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Clicker Question A 10 kg bag of rice weighs one-sixth as much on the moon than on earth because the moon’s gravity is one-sixth as much as the earth’s. If you tried to slide the bag horizontally across a smooth table to a friend, is it one-sixth as easier on the moon than on earth? (ignore friction) A) Yes B) No Answer: B No! The same horizontal force is needed, since the mass (inertia) of the bag is the same.
Towards Newton’s Second Law of Motion… (i) Acceleration is created by a net force Eg. Kick a soccer ball: what forces acting, causing what motion? First: accelerates from rest (i.e velocity from 0 to finite) due to your sudden push. While in air: velocity continues to change - eventually falls to the ground due to the (more gradual) force of gravity. Acceleration ~ net force ~ means, “directly proportional to” Twice the force on same object, gives twice acceleration

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Towards Newton’s Second Law of Motion… (ii) Mass resists acceleration Acceleration ~ __ 1 _ mass Eg. The same force applied to twice the mass gives half the acceleration Newton’s Second Law Puts (i) and (ii) together: The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object. F net a = m Often stated as F net = ma
Newton’s Second Law: Note about direction An object accelerates in the direction of the net force acting on it. Eg. Drop a ball – it accelerates downward, as force of gravity pulls it down Eg. We considered last time throwing a ball upward. When the ball is thrown upward, what is the direction of its acceleration (after leaving your hand)? Acceleration is downward (gravity) – so the ball slows down

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## This note was uploaded on 07/18/2011 for the course PHYS 10 taught by Professor Goldberg during the Fall '11 term at CUNY Hunter.

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Lec3 - Physics 101 Today: Chap 4 - Newtons Second Law Will...

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